The present invention relates to a method of detecting false-locking of a reference signal, provided by a local receiver oscillator, onto a digitally quadrature modulated signal with multiple levels and phase offset, and to a method of coherent demodulation using this method of detecting false-locking. In one particular implementation of these methods it is possible to establish the phase of the carrier of the signal to be demodulated. The invention further relates to devices for implementing these methods.
One example of digital modulation with multiple levels and phase offset is quadrature phase-shift keying with phase offset, known as OQPSK (offset quadrature phase shift keying), in particular modulation by shifting phase, with phase offset, on two carriers, each at two levels, known as MDP4. In direct modulation at microwave frequencies, two bit streams phase offset in time are mixed with two microwave carriers in quadrature, then added and transmitted. The two carriers are provided from the same microwave oscillator.
On reception, the signal to be demodulated is filtered if need be, then mixed with two receiver carriers in quadrature, which are theoretically of the same frequency as the transmitter carriers and are in phase with the latter (coherent demodulation). The baseband signals are recovered at the outputs of the mixers and correspond to the initial bit streams.
To effect phase-lock of the receiver oscillator, carrier recovery systems are customarily used, such as conventionally the COSTAS loop and the LECLERT & VANDAMME loop. These loops are stable when the transmitter oscillator and the receiver oscillator are in phase. However, they also have other stable points. Thus the COSTAS loop is stable for: EQU F.sub.ole -F.sub.olr =.+-.nD/4
and the LECLERT & VANDAMME loop is stable for EQU F.sub.ole -F.sub.olr =.+-.nD/(4.k),
where F.sub.ole and F.sub.olr are the frequencies of the local transmitter oscillator and the local receiver oscillator respectively, n and k are integers, and D is the symbol rate, that is to say the word rate on each channel.
When a point of stability is found at which the receiver frequency is different from the transmitter frequency, "false-locking" has occurred.
In practice, because of the high drift, especially with temperature, of the oscillators which are used (the drift even exceeding the bandwidth of the carrier recovery systems, which bandwidth is generally less than 300 kHz ), it is necessary for the receiver to incorporate a slow search device which uses a ramp to follow the frequency drift of the local receiver oscillator, especially its temperature drift.
Many devices are already known to effect detection of false-locking. In particular the applicants have proposed a method of detecting false-locking in their French patent application published under the No. 2 620 885, relying upon observation of the opening along the time axis of the eye diagram (the diagram observed on an oscilloscope synchronized by the clocks of the bit streams after recovering the baseband signal but before regeneration). When the carrier recovery loop is on a point of stability, the eye is open. However the opening of the eye along the time axis is smaller when false-locking occurs. That patent application proposes scrutinizing the eye to detect its closure.
Another system which is used is for the receiver to recognize a frame locking word. Such a system has some problems however. It necessitates the presence of a card for generating the frame locking word and of a card for recognizing the word. If either of the cards is omitted, detection is no longer provided. Furthermore, depending on the position of the bits of the locking word in the frame, it can happen that the locking word can be recovered at false-locking points.